MEMS enclosure

ABSTRACT

A low-cost, high-performance, reliable micromirror package ( 300 ) that replaces the ceramic substrate in conventional packages with a printed circuit board substrate ( 30 ) and a molded plastic case ( 33 ), and the cover glass with a window ( 36 ), preferably an optically clear plastic window. The printed circuit board substrate ( 30 ) allows for either external bond pads or flex cable connection of the micromirror package to the projector&#39;s motherboard. These packages support flexible snap-in, screw-in, ultrasonic plastic welding, or adhesive welding processes to overcome the high cost seam welding process of many conventional packages.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the packaging of semiconductorchips and particularly to that of micro-electrical mechanical systems(MEMS) such as micromirror devices.

[0003] 2. Description of the Related Art

[0004] Packaging is a critical part of producing a high-performanceMICROMIRROR for use in optical applications. Typically in a micromirror,the mirrors land on the substrate surface of the device. To avoid themirrors from sticking, it is necessary that the micromirror be packagedin a controlled environment, with minimum amounts of moisture,adhesives, dust, and other contaminants. In order to provide such anenvironment, hermetically sealed packages are often used. However, thecost of such packages and the negative impact on the assembly process isextremely high.

[0005] The packaging of micromirror chips for use in projection displayand other electro-optical applications has continued to present a costbarrier that contributes to higher prices for these products. A lowercost micromirror package is required to reduce the cost of thesehigh-resolution, digital projectors. Today's micromirror packages aremostly built on custom designed ceramic substrates and have expensiveglass covers (lids), which are seam welded or fixed in place with anadhesive. These packages are not only expensive, but they require a lowthroughput process that reduces the product cycle time.

[0006]FIG. 1 is a drawing of a welded package. This is a hermeticallysealed package that consists of a ceramic substrate 10 with a built-inKovar lid-attaching ring 11 on its surface and a lid assembly, whichconsists of a Kovar frame 12 with built-in an optical quality glasswindow 13. In the assembly process the micromirror die 14 is attached tothe substrate to provide both mechanical and thermal stability, as wellas an electrical ground plane. The micromirror leads are bonded to padson the substrate 10 which extend to external package pads on the edgesor bottom of the package. The package is filled with an inert gas andthen the lid assembly is seam welded at the two mating surfaces 15between the lid frame 12 and lid-attaching ring 11.

[0007] The adhesive sealed package shown in FIG. 2 has been used inplace of the welded package to lower the cost and improve themanufacturing throughput. This package is similar in that it has aceramic substrate 20 but the cover glass 21 (lid) is a single piece ofoptical quality glass. In this case, the micromirror 22 is mounted andbonded out in the substrate's 20 cavity and then the cover glass 21 isattached using an adhesive. The main advantages are that the assemblyprocess is somewhat simplified, which improves the manufacturingthroughput and the glass covers 21 are simple pieces of glass that canbe sourced from various vendors.

[0008] The micromirror packages discussed above perform very well butare too expensive and tend to limit cost reduction efforts due to theirhigh cost material and labor content. What is needed is a simplemicromirror packaging approach that is low cost, easily assembled, andreliable. The invention disclosed herein fulfills this need.

SUMMARY OF THE INVENTION

[0009] This invention discloses a low cost, high performance, reliablemicromirror package that overcomes many of the shortcomings of theconventional ceramic packages commonly used. This approach replaces theceramic substrate with a printed circuit board substrate, the ceramiccase with a molded plastic case, and the cover glass with an opticalquality plastic window or lid. The printed circuit board substrateallows for either external bond pads or flex cable connection of themicromirror package to the projector's motherboard. These packagessupport flexible snap-in, screw-in, ultrasonic plastic welding, oradhesive welding processes to overcome the low throughput, high cost,seam welding process of many conventional packages.

[0010] Other advantages include the following:

[0011] 1. Requires no seam welding,

[0012] 2. readily supports assembly automation,

[0013] 3. uses multiple source, commodity piece parts,

[0014] 4. light weight,

[0015] 5. has built-in thermal plane on the bottom of the printedcircuit board substrate,

[0016] 6. supports standard chip connection methods, such as edge pad,grid-pad, or flex cable, and

[0017] 7. lower cost.

DESCRIPTION OF THE VIEWS OF THE DRAWINGS

[0018] The included drawings are as follows:

[0019]FIG. 1 is a perspective view of a conventional welded hermeticallysealed, ceramic, micromirror package. (prior art)

[0020]FIG. 2 is a perspective view of a conventional epoxy sealed,ceramic, micromirror package. (prior art)

[0021]FIG. 3a is a perspective view of the low-cost, snap-on window,micromirror package of a first embodiment of this invention.

[0022]FIG. 3b is a sectional view of one portion of the package of FIG.3a showing the lid retention mechanism.

[0023]FIG. 4 is an exploded view of the micromirror package in FIG. 3a.

[0024]FIG. 5a is a perspective view of the low-cost, adhesive attachedwindow, micromirror package of another embodiment of this invention.

[0025]FIG. 5b is a sectional side view of a portion of the package ofFIG. 5a showing the adhesively sealed lid.

[0026]FIG. 6 is an exploded view of the micromirror package in FIG. 5a.

[0027]FIG. 7 is perspective view of the low-cost micromirror package ofthis invention configured with flexible interconnect cables.

[0028]FIG. 8 is a system level diagram of a one-micromirror projectiondisplay system incorporating the low-cost micromirror package of thisinvention.

[0029]FIG. 9 is a system level diagram of a 3-micromirror projectiondisplay system incorporating three of the low-cost micromirror packagesof this invention.

DETAILED DESCRIPTION

[0030] This invention discloses a low-cost, easily assembled micromirrorpackage. The packages of this invention use fiberglass printed circuitboard substrates, molded plastic parts, and lightweight plastic windowsinstead of ceramic parts and glass windows. The details of the twoembodiments are discussed below.

[0031]FIG. 3a shows a first embodiment of the micromirror package 300 ofthis invention. The package is comprised of a printed circuit board(PCB) base (substrate) 30, a molded plastic case 33, a topwindow-retaining ring 37, and a plastic optical window 36, enclosing amicromirror 38. This invention also allows for the attachment of anoptical aperture in the window area of the package, which will preventstray light from entering around the edge of the package where it canbounce around and contaminate the light coming from the micromirrormirrors. The high cost ceramic used in many conventional packages isreplaced by the lower cost PCB 30 (example—FR4) and molded plastic 33assembly. The PCB substrate 30 contains circuit traces 31, which areused to bond out the micromirror's 38 leads to the outside by means ofeither side contacts 32 or a grid-pad matrix (not shown) on the bottomof the package 300. Bond wires 39 are shown connecting the micromirror38 to the PCB traces 31. A thermal ground plane is also included on thebottom surface of the PCB 30, which is in effect the bottom of thepackage 300. The plastic case 33 is molded around the PCB 30 substrateto form a seal at the bottom of the package. There is an O-ring matingflange 34, shown in FIG. 3b, located on the top of the plastic case 33.The plastic case 33 also has snap-pockets where the window 36mounting-clamp 37 attaches to the assembly.

[0032]FIG. 3b is a sectional view of one side of the package showing thetop window 36 mounting and clamping mechanism. As shown, the plasticcase 33 has snap-pockets 40 molded into it to contain the opticalplastic window-clamping ring 37. An O-ring 35 sits on the O-ring-sealflange 34 and then the optical plastic window 36 sits on top of theO-ring. Finally, the mounting clamp 37 is placed over and around theperimeter of the top window 36 and pressed down, compressing the O-ring35, locking the snap-hooks 41 into the snap-pockets 40 formed in themolded plastic case 33, sealing the top portion of the package.

[0033]FIG. 4 is an exploded view of the micromirror package of the firstembodiment of this invention. The PCB 30 and the molded plastic case 33are mated together in the mold when the plastic case is manufactured,with the PCB 30 becoming the bottom of the package. The micromirror 38is attached to the PCB 30 and bonded out to the circuit interconnecttraces 31, shown in FIG. 3a, using standard semiconductor processes. AnO-ring 35 is then placed on the O-ring-seal flange 34 surface, shown inFIG. 3a, of the package. Finally, the window 36, preferably an opticallyclear plastic window or a glass window, is placed on top of the O-ring35, the mounting clamp 37 is place over and around the edges of thewindow 36, and the snap-hooks of retaining ring 37 are snapped into thesnap-pockets 40 in the plastic case 33, compressing the O-ring, toprovide a lightweight, sealed assembly.

[0034]FIG. 5a is a drawing showing a second embodiment of themicromirror package 500 of this invention, which uses an adhesive toattach the optically clear top window 56. The package is comprised of aprinted circuit board (PCB) base 50, a molded plastic case 53 having anadhesive-seal flange (surface) 54, and an optical window 56, andencloses a micromirror 57. This package is similar to that of the firstembodiment except for the way the optical window 56 is attached. Thebottom of the package is a PCB 50 with circuit traces 51 bringing themicromirror leads out to edge pads 52 or to a bottom grid-pad matrix. Asin the earlier case, the micromirror is attached to the PCB and bondwires 58 are attached between the micromirror input/output pads and thePCB traces 51. A thermal ground plane is also included on the bottomsurface of the PCB 50, which is in effect the bottom of the package 500.The primary difference in this embodiment is that the O-ring matingflange of the earlier embodiment is replaced with an adhesive-sealflange 54, which is an integral part of the molded plastic case 53. Inthis case, the snap pockets in the plastic case are no longer required.This package has all the benefits of the earlier package; low-cost,lightweight, easy assembly, and good reliability.

[0035]FIG. 5b is a sectional view of the optical plastic window 56mounting technique for the package in the second embodiment of theinvention. As shown, the plastic case 53 has an adhesive-seal flange 54built into it. The adhesive 55 is dispensed on top of the seal flange 54surface and the plastic window 56 sits on top of the adhesive 55. Theadhesive is then activated and cured to bond the window 56 to the moldedcase 53 to provide a completely sealed package.

[0036]FIG. 6 is an exploded view of the low-cost micromirror package ofthe second embodiment of this invention. The PCB 50 and the moldedplastic case 53 are mated together in the mold when the plastic case ismanufactured, with the PCB 50 becoming the bottom of the package. Themicromirror 57 is attached to the PCB 50 and bonded out to the circuitinterconnect traces 51 (FIG. 5a) using standard semiconductor processes.An adhesive 55 is then dispensed on to the adhesive-seal flange 54surface of the molded plastic case 53 and the optically clear plasticwindow 56 is placed on top of the adhesive 55. Finally, the adhesive isactivated and cured to properly seal the package.

[0037]FIG. 7 shows the micromirror packages of this invention withflexible interconnect cables integrally built into the package. AlthoughFIG. 7 shows the first embodiment of the invention, either of the twopackages discussed above can be configured with flexible interconnectcables. The package shown in FIG. 7, with flex-cable interconnectcapability, is comprised of a printed circuit board base 30 with PCBleads 31, a molded case 33 with a snap connected optical plastic window36, a mounted micromirror 38, bonding wires 39 connecting themicromirror 38 to the PCB traces 31, and flexible cables 70 (two shown)with lead traces 72 and attached connectors 71. This configuration canhave up to 4 flex-cables. The connectors 71 are used to connect themicromirror(s) into a motherboard or other bus.

[0038]FIG. 8 shows a system level block diagram for a single micromirrorprojection display system. The system is comprised of a light source 80,a first condenser lens 81, a motor/color filter wheel assembly 82, asecond condenser lens 83, a low-cost micromirror in the package of thisinvention 84, a fixed or zoom projection lens 85, and a viewing screen86.

[0039] Another example of a high-brightness micromirror projectiondisplay, which uses three of the low-cost micromirror packages of thisinvention, is shown in FIG. 9. This system is comprised of a lamp (lightsource) and reflector assembly 90, a condenser lens 91, a turning mirror92, a total internal reflective (TIR) prism 93, three micromirrors (forred, green, and blue light) in the low-cost package of this invention94, color splitting/color combining prisms 95, a fixed or zoom lens 96,and a viewing screen 97.

[0040] While this invention has beer described in the context of twopreferred embodiments, it will be apparent to those skilled in the artthat the present invention may be modified in numerous ways and mayassume embodiments other than that specifically set out and describedabove. Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

What is claimed is:
 1. A MEMS device package comprising: a plastic casewith an O-ring-seal flange; a printed circuit board base in said plasticcase; an O-ring seal on said flange; a window on said O-ring seal; and awindow retaining ring connected to said case and pressing said window tosaid O-ring.
 2. The MEMS package of claim 1, wherein said printedcircuit board is molded into said plastic case.
 3. The MEMS package ofclaim 1, wherein said printed circuit board has a center pad forattaching said MEMS device.
 4. The MEMS package of claim 1, wherein saidprinted circuit board has circuit traces for connecting micromirrorsignals to external circuitry.
 5. The MEMS package of claim 1, whereinsaid plastic case has: corner mounting holes; and built-in retainer ringsnap-slots, said window retaining ring connected to said case via saidsnap-slots.
 6. The MEMS package of claim 1, wherein said printed circuitboard has a thermal ground plane on a bottom surface.
 7. The MEMSpackage of claim 6, wherein said printed circuit board circuit tracesconnect to edge pads around the perimeter of said package.
 8. The MEMSpackage of claim 6, wherein said printed circuit board circuit tracesconnect to a matrix of grid-pads on the bottom surface of said package.9. The MEMS package of claim 6, wherein said printed circuit boardtraces are connected to flex-cables with built-in connectors.
 10. Amethod of packaging a MEMS device, the method comprising the steps of:attaching a MEMS device to a printed circuit board base; bonding pads onsaid MEMS device to pads on said printed circuit board base; placing anO-ring on an O-ring-seal flange of a plastic case; placing a plasticwindow on top of said O-ring; placing a snap-on window retaining ring ontop of and around the perimeter of said window; and compressing saidsnap-on window retaining ring, window, and O-ring until said snap-onretaining ring hooks lock into snap-slots located in said plastic case.11. A MEMS package comprising: a plastic case with an adhesive-sealflange; a printed circuit board base in said plastic case and; a windowbonded to said adhesive-seal flange.
 12. The MEMS package of claim 11,wherein said printed circuit board is molded into said plastic case. 13.The MEMS package of claim 11, wherein said printed circuit board has acenter pad for attaching said micromirror.
 14. The MEMS package of claim11, wherein said printed circuit board has a thermal ground plane on abottom surface.
 15. The MEMS package of claim 11, wherein said printedcircuit board has circuit traces for connecting micromirror signals toexternal circuitry.
 16. The MEMS package of claim 15, wherein saidprinted circuit board circuit traces connect to edge pads around theperimeter of said package.
 17. The MEMS package of claim 15, whereinsaid printed circuit board circuit traces connect to a matrix ofgrid-pads on the bottom surface of said package.
 18. The MEMS package ofclaim 15, wherein said printed circuit board traces are connected toflex-cables with built-in connectors.
 19. A method of packaging amicromirror, said method comprising the steps of: attaching amicromirror to a printed circuit board base; bonding micromirror pads toprinted circuit board base pads; placing said printed circuit board in aplastic case; dispensing adhesive onto an adhesive-seal flange on saidplastic case; placing a window on top of said adhesive; and curing saidadhesive.
 20. A micromirror projection display comprising: a lightsource for providing a beam of light along a light path; a firstcondenser lens on said light path for receiving said beam of light; afilter assembly on said light path for receiving and filtering said beamof light from said first condenser lens; a second condenser lens forreceiving said filtered beam of light; a micromirror mounted in alow-cost molded plastic package with built-in printed circuit board baseand window for selectively modulating said filtered beam of light; and aprojection lens for receiving said modulated beam of light and focusingsaid modulated beam of light on an image plane.
 21. The micromirrorprojection display of claim 16, wherein said micromirror packagecontains flex-circuit interconnect cables with built-in connectors. 22.A high-brightness micromirror projection display comprising: a lightsource for providing a beam of light along a light path; a firstcondenser lens on said light path for receiving said beam of light; atotal internal reflective prism receiving said beam of light; acolor-splitting prism assembly receiving said beam of light from saidtotal internal reflective prism; three micromirrors mounted in alow-cost molded plastic packages with built-in printed circuit boardbases and windows for selectively modulating said filtered beam oflight; and a projection lens for receiving said modulated beam of thelight focusing said modulated beam of light on an image plane.
 23. Themicromirror projection display of claim 23, wherein said micromirrorpackages contain flex-circuit interconnect cables.